Read the entire thread and i still can't find an answer to this. I've never heard of a problem like this... Do you use anything else for your reservoir other than what you've mentioned?
Also, just as a recommendation, try to steer away from PPM measurements because of the different in scaling, and try to always use EC, it's just a more reliable way to measure.. And since you've been using the 500PPM = 1 EC scale, then... Maybe? For instance, I've had landrace strains that never liked going over 0.8 EC as they would show similar symptoms with the leaves curling down.
But the yellow on your leaves look like either a deficiency or lockout... Which only serves to confuse me.